The invention relates to a reaction-distillation apparatus and its use for carrying out a chemical reaction and fractionating the reaction mixture.
It also relates to a method of preparing an ether by reacting olefins with alcohols, wherein the reaction and distillation of the products formed, particularly with a view to separating them from non-reacted constituents, are carried out concurrently.
It has long been known that in the case of a balanced reaction, conversion of the reagents is limited by the thermodynamics. If an excess quantity of one of the reagents is introduced, conversion of the other reagents is increased. But, this method is often costly, since it requires an additional installation to recover the reagent which was introduced in excess.
An effective way of obtaining total conversion beyond the thermodynamic equilibrium is to apply the so-called reactive distillation method. This comprises carrying out in the same chamber the reaction, normally in the presence of a catalyst, and distillation, to separate the products from the other constituents as they are formed. This method is used, for example, for etherification reactions (U.S. Pat. No. 3,629,478; U.S. Pat. No. 4,847,430 and EP-B-8860).
U.S. Pat. No. 3,629,478 proposes that distillation plates be used and that the catalyst be disposed only in the liquid downcomer passages from those plates, avoid the disruptive effective of the vapor phase passing through the catalyst. However, the presence of the catalyst in those passages creates a pressure drop, such that the liquid tends to descend countercurrently into the ports provided for the vapor to pass onto the work table of each distillation plate. Thus, in this construction a large part of the liquid does not come into contact with the catalyst; this limits the effectiveness of the reaction-distillation apparatus as far as the reaction is concerned.
U.S. Pat. 4,847,430 describes the use of reaction-distillation zones where the catalytic bed alternates with the distillation zone. Passages through the catalytic bed which are reserved for the vapor phase avoid gas/liquid contact and limit the problem of pressure loss. On the other hand, physical liquid/vapor equilibrium is not established permanently. Hence one of the reagents in the catalytic zone may be exhausted with a consequent loss of effectiveness in the reaction, particularly since the reactive column operates under conditions close to chemical equilibrium.
EP-B-8860 proposes the use of a distillation column filled with a catalyst appropriate for the preparation of methyl tertiobutyl ether (MTBE), wherein the catalyst, according to the form which it takes, also at least partly fulfills the function of a packing for distillation, thus forming the MTBE and, at the same time, separating it from the other constituents present.
However, the overall structure of the packing containing the catalyst and its vertical piling-up in the column are unfavorable to gas/liquid contact and do not produce effective distillation. Furthermore, this type of column filling is expensive, since it includes the cost of the catalyst, the cost of making the catalytic packing, and the cost of the metal netting wound around the catalytic mat. Moreover, when the catalyst has to be changed, the metal netting must also be changed, thereby considerably increasing the cost of the operation.